1. Field of the Invention
The present invention relates to a driving apparatus employing a shape memory alloy, and more particularly to a driving apparatus that is capable of correcting for camera shake by exploiting expansion/contraction of a shape memory alloy.
2. Description of the Prior Art
A shape memory alloy (hereinafter referred to also as an “SMA”), even if it is plastically deformed by application of a force at a temperature below the temperature at which the martensitic transformation ends, recovers its original shape when heated to above the temperature at which the reverse transformation ends. By exploiting this shape memory effect, it is possible to build an actuator employing a shape memory alloy. In fact, diverse studies have been made in this field. For example, an article by Kuribayashi in “Systems and Control,” Vol. 29, No. 5, 1985, presents the results of a study on mathematical models and position/force control of control elements employing shape memory alloys.
FIG. 21 is a diagram illustrating the principle of a driving apparatus employing a shape memory alloy. In this figure is shown a driving apparatus 50 of a twin-SMA type. The twin-SMA-type driving apparatus 50 has shape memory alloy members 52 and 53 connected at both ends of a movable member 51. The shape memory alloy here is typically operated by the Joule heat that is generated when electric current is passed through it.
Specifically, when electric current is passed between both ends of a shape memory alloy, heat is generated, and thus the shape memory alloy recovers its original length. As a result, the shape memory alloy contracts, and its elastic modulus against tension increases. When the electric current flowing through the shape memory alloy is cut off, the heat dissipates, and thus the temperature of the shape memory alloy falls. As a result, the shape memory alloy becomes soft and easily deformable elastically.
Here, the force (hereinafter referred to as the “generated force”) Fd applied to the movable member 51 is given by formula (1) below, where Fp represents the generated force ascribable to the contraction of one of the shape memory alloy members, and Fm represents the reaction of the other.Fd=Fp−Fm  (1)
Hence, when electric current is passed between both ends 52a and 52b of the shape memory alloy member 52 (hereinafter referred to also as the “pSMA”) and no electric current is passed between both ends 53a and 53b of the shape memory alloy member 53 (hereinafter referred to also as the “mSMA”), the generated force Fd changes from a balanced state to a state deviated to the positive side.
In this state, the generated force Fp ascribable to the pSMA 52 on one side, which has contracted by being heated, makes the mSMA 53 on the other side expand. This makes the movable member 51 move in the positive (+) direction. When the movable member 51 travels a predetermined distance, the generated force Fd becomes equal to 0 (Fp=Fm). Thus, the driving apparatus 50 comes to rest. By applying additional electric current to pSMA 52, the movable member 51 can be driven farther in the positive (+) direction.
Likewise, when electric current is passed between both ends 53a and 53b of the mSMA 53 and no electric current is passed between both ends 52a and 52b of the pSMA 52, while the mSMA 53 contracts, the pSMA 52 expands. Thus, the movable member 51 moves in the negative (−) direction.
FIG. 22 shows the temperature hysteresis observed in the movement of the movable member 51 when the driving apparatus 50 is operated. In this figure, along the vertical axis is taken the position of the movable member 51, and along the horizontal axis are taken the temperatures of the shape memory alloy members (pSMA and mSMA) 52 and 53. When electric current is passed through the pSMA 52 and no electric current is passed through the mSMA 53, the movable member 51 moves as indicated by an arrow A1. Meanwhile, when the temperatures of the pSMA 52 and the mSMA 53 are t2 and t1, respectively, the movable member 51 is located in the middle position.
When electric current is passed through the mSMA 53 and no electric current is passed through the pSMA 52, the movable member 51 moves as indicated by an arrow A2. Meanwhile, when the temperatures of the pSMA 52 and the mSMA 53 are t1 and t2, respectively, the movable member 51 is located in the middle position.
By energizing and de-energizing the shape memory alloy members 52 and 53 at short time intervals, the movable member 51 can be moved around the middle position as indicated by broken lines B1. Thus, by incorporating the driving apparatus 50 in a camera and making it move an optical system in response to and in the opposite direction to camera shake resulting from unstable holding of the camera, it is possible to correct for the camera shake.
Camera shake that occurs during photographing using a camera typically has a frequency of from a few Hz to 10 Hz. Therefore, when the driving apparatus 50 is used for camera shake correction, it needs to have sufficiently fast response to follow a shake of at least 10 Hz. FIG. 23 is a diagram showing the response characteristics of the driving apparatus 50 observed when the shape memory alloy members 52 and 53 are energized and de-energized at a frequency of 10 Hz. In this figure, along the vertical axis is taken the position of the movable member 51, and along the horizontal axis is taken the lapse of time (in msec). The broken line indicates the ideal movement (target position) without a delay in response, and the solid lines indicate the actual movement at different ambient temperatures around the driving apparatus 50, namely 25° C., 50° C., and 60° C.
Here, the shape memory alloy members 52 and 53 are made of a Ti—Ni alloy, of which the operating temperature T is 65° C. The operating temperature T denotes the average temperature at which the shape memory alloy is operated, and is given by T=(t1+t2)/2 (see FIG. 22).
FIGS. 24 and 25 show the temperature hysteresis observed in the movement of the movable member 51 at ambient temperatures of 25° C. and 60° C., respectively. In these figures, along the vertical axis is taken the position of the movable member 51, and along the horizontal axis is taken the temperatures of the shape memory alloy members (pSMA and mSMA) 52 and 53. These figures show that, at an ambient temperature of 25° C. around the driving apparatus 50, the width tw of the temperature hysteresis is small and the distance traveled is long. This makes it possible to make the movable member 51 move in such a way that it considerably precisely follows the input electric current.
However, at an ambient temperature of 50° C. or 60° C. around the driving apparatus 50, it is not possible to obtain the desired amplitude, and a long delay in phase results from temperature hysteresis. This makes it impossible to follow the input electric current, causing a maximum error Dmax of 50% relative to the target position.
Specifically, for example, when the pSMA 52 is energized, heat dissipation from mSMA 53 is insufficient, and thus the elastic force of the mSMA 53 surpasses the contractive force of the pSMA 52. Subsequently, as heat is dissipated from the mSMA 53, the pSMA 52 contracts with a delay, and the movable member 51 moves in the positive (+) direction. However, before the movable member 51 reaches the target position, the mSMA 53 is energized, making it impossible to produce the desired amount of correction. This leads to the problem of insufficient camera shake correction at 50° C., which is generally considered the upper limit of the temperature range in which a camera or the like is guaranteed to operate correctly.
Moreover, there is a possibility that, even when one shape memory alloy member is de-energized, its temperature continues to rise, and a possibility that, even when the other shape memory alloy member ends being heated, its temperature does not sufficiently fall. As a result, the shape memory alloy members 52 and 53 are overheated to an excessively high temperature. If the shape memory alloy is heated to above 100° C. and kept at that temperature for a few tens of seconds, it no longer retains its original shape. This leads to the problem of the driving apparatus 50 becoming inoperative, or the driving apparatus 50 being even destroyed.